The invention relates to a method for preparing a pigment concentrate, more in particular to a pigment concentrate suitable for preparing a powder coating.
It should be noted that powder coatings are coating types which are applied by fluidized-bed dip coating or electrostatic spraying. This is followed by baking, as a result of which the powder forms a homogeneous film.
The powder coating is formed by a paint manufacturer by one or more pigments being mixed with a coating system (consisting of a resin and, where required, a hardener) and generally one or more additives and/or fillers. The mixture obtained is then homogeneously fused (where applicable, at a temperature below the cross-linking temperature) in an extrusion device and is extruded. The extrudate is then cooled to room temperature as rapidly as possible and is finely ground to obtain a powder which can be used as a powder coating.
The drawback of such a powder coating, however is that the powder formed may still have a suitable tinting strength reserve, tinting strength reserve being understood as meaning an amount of pigment whose staining power is not utilized. This can e.g. lead to color differences between batches.
GB-A-1197053 discloses a process for preparing pigment compositions comprising mixing an aqueous mass of a pigment and an aqueous emulsion or solution of a carrier (polyolefin, polyvinyl chloride, polyethylene oxide derivative, rosin or rosin derivative) to form an unagglomerated mixture and spray drying said mixture.
U.S. Pat. No. 3,759,864 discloses a process for preparing pigmented solid particles of film-forming polymer by volatilizing the liquid continuous phase of a pigmented dispersion of film-forming polymer.
BE-A-842049 discloses a process for preparing pigment compositions by mixing an aqueous suspension of polymer containing 30-70 wt % of polymer with an aqueous suspension of a mineral pigment containing at least 30 wt % of pigment, and spray drying the mixture at 30-200xc2x0 C.
WO-A-95/31507 discloses a method for preparing a pigment concentrate, which is characterized in that pigment particles are comminuted in the presence of an aqueous binder and preferably of a dispersant to form a stable paste or suspension, the paste or suspension formed is subjected to a drying treatment, and the pigment concentrate is collected. The binder is preferably a resin having a glass transition temperature of 25-55xc2x0 C. A most preferred binder is a saturated polyester resin having an acid value of from 20 to 55.
The prior art however still shows the above-mentioned drawback.
There was thus a need for an improved method for preparing a pigment concentrate suitable for powder coatings, and particularly a need for an improved composition for use in said method.
The method of the invention for preparing a pigment concentrate suitable for preparing a powder coating is characterized in that pigment particles are comminuted in the presence of an aqueous binder essentially consisting of a polyester having a glass transition temperature of at least 60xc2x0 C. and an acid value of at least 60, to form a stable paste or suspension, the paste or suspension formed is subjected to a drying treatment, and the pigment concentrate is collected.
The drying treatment applied preferably consists of spray drying; other drying treatments such as those employing a drying drum or a belt dryer for example may however also be used.
It should be noted that a stable paste or suspension has been obtained once the comminuted pigment particles no longer settle in the paste or suspension within a specific rest period of the paste or suspension. Usually, a period of 5-30 min. is chosen for this purpose.
During drying, only the amount of water present in the aqueous binder used is evaporated. The present process is therefore particularly environment-friendly, while it is moreover economically advantageous, since no material has to be recovered.
In the method according to the invention, there is preferably, in addition to the binder, no dispersant present. It was previously thought (e.g. in WO-A-95/31507) that a dispersant would make it easier to obtain a stable suspension or paste. Normally, thus, the pigment concentrate will contain 35-65% of pigment, the remainder being (anhydrous) binder.
Expediently, a binder free of cosolvent is employed. In practice it was found, however, that a cosolvent content up to approximately 5% in the binder to be used gives fairly acceptable results. The best results are obtained, however, with a cosolvent content of 0-1%.
In the method according to the invention, the binder used is preferably a resin which has a glass transition temperature of at least 60xc2x0 C., more preferably at least 65xc2x0 C., and preferably at most 80xc2x0 C., more preferably at most 70xc2x0 C. Such a material is still found to be readily fusible in the extrusion device during the subsequent formation of the powder coating. It should be noted in this context that the temperature used in the extrusion device is usually from 110 to 130xc2x0 C.
Binders for use in the pigment concentrates of the invention essentially consist of polyester resins. Preferably, a saturated polyester resin is used. A particularly suitable polyester resin has an acid value between 60 and 90, more preferably of at least 70.
As stated above, in the present invention, pigment particles are comminuted with the formation of a stable paste or suspension. It was found in practice that a paste or suspension is stable if a particle size of less than 15 xcexcm, preferably less than 10 xcexcm, is obtained; such a particle size is preferably produced with the use of a bead mill. Obviously, the pigment particles must always be smaller than the thickness of the coating film ultimately to be formed. Moreover, the particle size is important for the gloss of the coating film to be formed, in the sense that the gloss is partly determined by the particle size of the pigment particles and their dispersity.
The invention was found to be particularly suitable for organic pigments; this is because it is very difficult to disperse such pigments during the manufacture of a powder coating. It was found, however, that as a result of them being used to make a pigment concentrate, such problems can be completely overcome.
Organic pigments which can be used, for example, are carbon black, phthalocyanine green, phthalocyanine blue, organic red and organic yellow.
The invention also relates to a method for forming a powder to be used as a powder coating, by mixing a pigment with a thermoplastic or thermosetting plastic, where applicable a hardener, and additives, by blending, melting, extruding and finely grinding the mixture into a powder, which method is characterized in that the pigment used is a pigment concentrate prepared according to the method disclosed above.
The invention further relates to a method for coating a substrate with a powder coating by mixing pigment particles with a thermoplastic or thermosetting plastic, where applicable a hardener, and additives, melting and extruding the mixture formed, comminuting the extrudate formed into a powder, applying the powder obtained to a substrate in the form of a thin layer and melting said thin layer to form a coating, which is characterized in that the pigment particles used are pigment concentrate particles obtained according to the method disclosed above.
The invention is particularly applicable to powder coating compositions based on polyester or epoxy-polyester resins.